Isoflavones and many derivatives thereof possess a wide range of important biological properties including estrogenic effects. Isoflavanoids found in soy, such as genistein and daidzein, have also attracted interest as dietary phytoestrogens that might be effective for the treatment of hormone-dependent conditions and diseases. In examining the impact of the estrogenic activity of soy isoflavones (commonly referred to as phytoestrogens), one needs to consider not only the isoflavones and their conjugates that are ingested, but also biologically active metabolites that might be generated in vivo. Daidzein can be converted to the corresponding chromane S-equol, a compound with greater estrogenic activity than its precursor. Setchell et al., J. Nutrition, 2002, 132(12): 3577-3584. This reductive metabolic conversion is the result of the action of equol-producing gut microflora found in a small proportion of the human population who are known as “equol producers.”
Equol was first isolated from a pregnant mare's urine in 1932 and was subsequently identified in the plasma of red clover-consuming sheep. In 1982, equol was identified in human urine. Equol has a chiral center and therefore can exist in two enantiomeric forms. It has been recently established that S-equol is the exclusive enantiomer produced by intestinal bacterial flora in the metabolic reduction of isoflavones ingested by humans. Setchell et al., American Journal of Clinical Nutrition, 2005, 81:1072-1079.
The structure of S-equol is shown below.

S-equol, R-equol, non-racemic mixtures of S- and R-equol (“equol”); compositions of equol; processes for the preparation of equol; and methods of using equol are described in U.S. Pat. No. 8,716,497 (filed Sep. 10, 2012); U.S. Pat. No. 8,048,913 (filed Sep. 14, 2009); U.S. Pat. No. 7,960,432 (filed Jul. 3, 2008); U.S. Pat. No. 7,396,855 (filed Jul. 24, 2003); U.S. Pat. No. 8,263,790 (filed Jun. 1, 2011); U.S. Pat. No. 7,960,573 (filed May 4, 2009); U.S. Pat. No. 7,528,267 (filed Aug. 1, 2005); U.S. Pat. No. 8,668,914 (filed Jul. 31, 2009); U.S. Pat. No. 8,580,846 (filed Aug. 18, 2006); U.S. Pat. No. 8,450,364 (filed Apr. 9, 2012); and U.S. Pat. No. 8,153,684 (filed Oct. 2, 2009); each of which is incorporated by reference in its entirety.
Drugs utilized in the preparation of pharmaceutical formulations for commercial use must meet certain standards, including GMP (Good Manufacturing Practices) and ICH (International Conference on Harmonization) guidelines. Such standards include technical requirements that encompass a wide range of physical, chemical, and pharmaceutical parameters.
For example, a drug utilized for the preparation of pharmaceutical formulations should meet an acceptable purity. There are established guidelines that define the limits and qualification of impurities in new drug substances produced by chemical synthesis; that is, actual and potential impurities most likely to arise during the synthesis, purification, and storage of the new drug substance. Guidelines are instituted for the amount of allowed degradation products of the drug substance, or reaction products of the drug substance with an excipient and/or immediate container/closure system. In the guidelines, moisture specifications must be met.
Stability is also a parameter considered in creating pharmaceutical formulations. A stable drug product will ensure that the desired chemical integrity of drug substances is maintained during the shelf-life of the pharmaceutical formulation, which is the time frame over which a product can be relied upon to retain its quality characteristics when stored under expected or directed storage conditions. Different factors, such as light radiation, temperature, oxygen, humidity, and pH sensitivity in solutions, may influence stability and may determine shelf-life and storage conditions.
Bioavailability is also a parameter to consider in drug delivery design of pharmaceutically acceptable formulations. Bioavailability is concerned with the quantity and rate at which the intact form of a particular drug appears in the systemic circulation following administration of the drug. The bioavailability exhibited by a drug is thus of relevance in determining whether a therapeutically effective concentration is achieved at the site(s) of action of the drug.
Many pharmaceutical solids exhibit polymorphism, which is generally defined as the ability of a substance to exist as two or more crystalline phases (i.e., different arrangements and/or conformations of the molecules in a crystal lattice). Thus, polymorphs are different crystalline forms of the same substance in which the molecules have different arrangements and/or conformations of the molecules. As a result, the polymorphic solids have different physical properties, including those due to packing, and various thermodynamic, spectroscopic, interfacial, and mechanical properties.
Packing properties include the polymorph's molar volume and density, refractive index, thermal and electrical conductivity, and hygroscopicity. Thermodynamic properties include the polymorph's melting and sublimation temperatures, internal structural energy, enthalpy, heat capacity, entropy, free energy and chemical potential, thermodynamic activity, vapor pressure, and solubility. Spectroscopic properties include the polymorph's electronic transitions (ultraviolet-visible absorption spectra), vibrational transitions (infrared absorption spectra and Raman spectra), rotational transitions (far infrared or microwave absorption spectra), and nuclear spin transitions (nuclear magnetic resonance spectra). Kinetic properties include the polymorphs's dissolution rate, rates of solid state reactions, and stability. Surface properties include the polymorph's surface free energy, interfacial tensions, and shape habit. Mechanical properties include the polymorph's hardness, tensile strength, compactibility (tableting), as well as handling, flow, and blending.
The various different chemical and physical properties of polymorphic forms can have a direct effect on the processing and/or manufacturing of the drug substance and the drug product. For example, the solid-state properties of the active ingredient will likely be critical to the manufacture of the drug product, particularly when it constitutes the bulk of the tablet mass in a drug product manufactured by direct compression. With respect to pharmaceutical processing, one of the most relevant factors for polymorphs is whether it can be consistently manufactured into a drug product that conforms to applicable in-process controls and release specifications.
Polymorphic forms of the drug substance can undergo phase conversion when exposed to a range of manufacturing processes, such as drying, milling, micronization, wet granulation, spray-drying, and compaction. Exposure to environmental conditions such as humidity and temperature can also induce polymorph conversion. The extent of conversion generally depends on the relative stability of the polymorphs, kinetic barriers to phase conversion, and applied stress. The most thermodynamically stable polymorphic form of a drug substance is often chosen during development based on the minimal potential for conversion to another polymorphic form and on its greater chemical stability. Therefore, it is important to find not only the most thermodynamically stable polymorphic form of a drug substance, but also the polymorphic form that is most amenable to a large-scale manufacturing process.
Crystallization and drying are two of the most difficult operations in a scale-up process, especially when the compound is polymorphic and can form a hydrate or solvates. In the case of hydrates, removal of water from the crystal lattice requires particularized conditions and is very much dependent on the temperature and humidity history of the sample. The mobility of water among various components in a formulation must always be considered. Water interacts with pharmaceutical solids at virtually all stages of manufacture. The amount of moisture absorbed by drugs and excipients effects the flow, compression characteristics, and hardness of granules and tablets. Thus, it is very important to establish that the desired form is produced and to minimize batch-to-batch variability.
In addition, it is desirable to regulate crystal polymorphism of a chemical substance, or an ingredient thereof, because differences in crystalline form may affect properties of the drug such as performance of the preparation, bioavailability, and stability. The difference in the physical properties of different solid state forms results from the orientation and intermolecular interactions of adjacent molecules or complexes in the bulk solid. Accordingly, polymorphs are distinct solids sharing the same molecular formula, while having distinct physical properties, which may be advantageous relative to other solid state forms of the same compound or complex.
Equol has been isolated in a crystal form in the literature. Liang et al., X-Ray Single-Crystal Analysis of (−)-(S)-Equol Isolated from Rat's Feces, Chemistry and Biodiversity 2005, 2, 959-963. However, as provided on page 960, the only crystal form disclosed is a hydrate of S-equol. Hydrate forms of crystalline materials are generally known to be less stable than anhydrous crystalline forms in certain conditions. Thus, there is a need in the art to provide a pure anhydrous crystalline form of S-equol having improved stability.